Coke cans focus sound waves beyond the diffraction limit

Coke can lens
The Coke can acoustic lens, surrounded by speakers, can focus sound waves to a space 1/25th the size of a wavelength. Image credit: Fabrice Lemoult

( -- When trying to focus sound waves into as small an area as possible, scientists run into a fundamental limit called the diffraction limit. That is, when sound waves are focused into a region smaller than one wavelength, the waves begin to bend and spread out. Recently, scientists have designed complex acoustic metamaterial lenses in an attempt to overcome the diffraction limit, but now a new study shows that this can be done using much simpler materials - specifically, 49 empty Coke cans.

To build the acoustic lens, physicists Geoffroy Lerosey, Fabrice Lemoult, and Mathias Fink at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech) assembled a 7x7 array of empty Coke cans with the tabs pulled off.

Then, the scientists surrounded the Coke can array with eight computer speakers. When they turned the speakers on to play a single tone, the sound waves traveled around and inside the cans, causing the cans to collectively oscillate like organ pipes. As a whole, the lens generated a variety of patterns, some of which emanated from the can openings, which are much smaller than the of the sound waves.

The small waves are similar to , which can reveal details smaller than a wavelength and be used to focus sound. If researchers can capture evanescent waves, they can beat the . However, evanescent waves only exist very close to an object’s surface because they fade very quickly, making them difficult to capture. Previously, scientists have used acoustic metamaterial lenses to amplify the evanescent waves in order to make them easier to capture.

Here, the researchers figured out a way to amplify and capture the evanescent-like waves coming from the soda cans using a method called “time reversal.” They recorded the sound above a single can with a microphone, and then played this sound backwards through the speakers. The resulting sound waves amplify the sound above the can from which the original sound came from, and cancel out the sound everywhere else.

As this single can continues to resonate, sound waves inside the can become scattered. While the normal sound waves scatter and disappear quickly, the evanescent-like waves take longer - about a second - to scatter out of the can. That’s enough time to allow the evanescent-like waves to build up into a highly focused spot of just a few centimeters, or about 1/25th the space of the meter-long wavelength of the original acoustic wave. Such focus is significantly beyond the diffraction limit.

"Without being too enthusiastic, I can say [our work] is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey told Nature News. (Sub-wavelength focusing in the near field, where different wave behavior dominates, has already been demonstrated.)

By showing that a simple Coke can can focus beyond the diffraction limit, the study could have applications in providing energy for tiny electromechanical devices, among other uses.

Explore further

Novel metamaterial vastly improves quality of ultrasound imaging

More information: F. Lemoult, et al. Physical Review Letters. To be published.

via: Nature News and Physics World

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Citation: Coke cans focus sound waves beyond the diffraction limit (2011, July 12) retrieved 19 September 2019 from
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Jul 12, 2011
Yet more proof the 2nd law of thermodynamics is just religion.

Jul 12, 2011
Perhaps the same concept applied at the nanometer scale could be used to harvest ambient noise ?

Jul 12, 2011
Yet more proof the 2nd law of thermodynamics is just religion.


Jul 12, 2011
Yet more proof the 2nd law of thermodynamics is just religion.

please do explain.

Jul 12, 2011
Yet more proof the 2nd law of thermodynamics is just religion.

please do explain.

Indeed! I cannot wait to hear this person's explanation!!!

Jul 12, 2011
An interesting application of this concept is ultrasound examinations that give four times sharper images than the wavelength of the sound.
This means lethal* deformities of a fetus can be identified early, and this will in turn make late-term abortions practically extinct. (*some fetuses are sadly so messed up they will not survive birth. Since current ultrasound is too imprecise to pick up all such cases, sometimes this leads to traumatic late-term abortions).

I can also see apps for detecting tumors, screening materials for flaws (remember those brittle ceramic tiles used by the space shuttle)... only imagination sets limits to what can be done.

Jul 12, 2011
Who drank the Coke?

Jul 12, 2011
Weren't there any effervescent waves?

Jul 13, 2011
I hope no one connects this contraption up to an anti-mass spectrometer, or we'll have ourself a resonance cascade.

Jul 13, 2011
I hope no one connects this contraption up to an anti-mass spectrometer, or we'll have ourself a resonance cascade.

Thank heavens for that hazard suit.

Jul 13, 2011
I hope no one connects this contraption up to an anti-mass spectrometer, or we'll have ourself a resonance cascade.

Thank heavens for that hazard suit.

And for such thorough training simulations.

Jul 13, 2011
Oh, I get it.

"just religion" = "something I don't really understand"

Jul 13, 2011
The second "LAW" of thermodynamics IS religion.

Speech and writing affects thought patterns, right from the time you began to understand the patterning of 'noise and shapes' into 'speech and writing'. The idea of calling a THEORY a LAW is absolutely insane. patently so. It is all theory, there are no laws in science. Laws are for social situations, cultural situations where one group wishes to and does impose a code of conduct upon all personages within the group. They also give out penalties for violation, up to and including execution. Physical imposition upon all members of the given society. Behavioral control via punishment.

So to call the THEORY of thermodynamics (which is Newtonian not quantum) a LAW is completely bug-fornicate insane and ha zero to do with real science. It is an engineering term, this law thing. For engineers are to make things--and never create new fundamentals.

Basically, screw off with the LAW crap. Only an illiterate would cal it a 'law'.

Jul 13, 2011
It's amazing how so many solutions to life's problems can be found under our noses. Next thing we know, they'll discover that the cure for all cancers is to lick the bottom of some specific toad species...

Jul 13, 2011
Incidentally, did the people that drank the cola get listed in the paper as contributors?

Jul 13, 2011
@KBK: Your point re. the difference between theory and law is valid, if a little pedantic. Ok... a LOT pedantic.

But isn't it fair to draw a distinction between religious tenets that are LITERALLY articles of faith, and thermodynamic laws (or whatever you want to call them) that have so far stood up to every experimental test that's been thrown at them?

Jul 14, 2011
It is indeed very sad to see people get caught in such crackpottery as this. He's either trolling, trying to be original at any cost or has really lost his mind.

No laws in science... pfffffff now THAT'S BULLSHIT.

Jul 15, 2011
Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third, they are in thermal equilibrium with each other.

First law of thermodynamics: Energy can be neither created nor destroyed. It can only change forms.

Second law of thermodynamics: The entropy of an isolated system tends to a maximum.

The third law of thermodynamics: As temperature approaches absolute zero, the entropy of a system approaches a minimum.

I think I see what KBK is trying to say: The entropy of religion tends to a maximum. Clever!

Jul 17, 2011
something to think about:
1) Will this discovery make the "pimp my ride" sub-woofers even more unbearable?

2) If you have a sub woofer in your vehicle don't throw the empty beer cans into the back seat.

3) I assume, that unless they get research funding from Coca-Cola, the next step is to check with empty French wine bottles.

Jul 18, 2011
"The 2nd law of thermodynamics is just religion." means that it is false, and still a lot of people believe it to be true.

And that should be obvious to anyone. With all other physical laws being time-reversal invariant (or time-charge-parity reversal invariant at worst), it is obvious that a system where that "law" is not valid can easily be defined by reversing velocities (and charges, and positions perhaps) in some other system where it is valid.

The 2nd law is just a statistical conclusion, it may apply to our immediate surroundings, but it is impossible to be an absolute and general "law".

I do not see how this article relates to this though.

Jul 22, 2011
As a Texan, I must insist that they retest with Dr. Pepper cans. There is no reason for Coca-cola to get all the free advertising here.
Also, as far as that goes, why did they not use Mountain Dew cans? After all, it is a well-known staple of geek culture.

Jul 25, 2011
If this had any application to thermodynamics, you might actually be right. How sad for you that it doesn't.

"Yet more proof the 2nd law of thermodynamics is just religion." - Koch

Ok, so what is phonon theory? Here we witness perfect anti-causality creating a time-reversed phase-conjugate longitudinal wave from non-linear media. Disorder reconstructs perfect order.

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